In recent years, directed self assembly has emerged as a useful means of organizing solution-synthesized nanostructures to create lithographic features and for a wide variety of other applications. For example see Thurn-Albrecht et al., “Ultrahigh Nanowire Arrays Grown in Self-Assembled. Diblock Copolymer Templates”, Science, 290, 2126, (2000). Black et al., “Integration of Self-Assembled Diblock Copolymers for Semiconductor Capacitor Fabrication,” Applied Physics Letters, 79, 409, (2001) and Akasawa et al., “Nanopatterning with Microdomains of Block Copolymers for Semiconductor Capacitor Fabrication,” Jpn. J. Appl. Phys., 41, 6112, (2002).
In many directed self assembly applications, molecular interactions drive phase separation into domains; wherein immiscible polar and nonpolar materials are concentrated. Of particular interest in directed self assembly applications are thin films of block copolymers that have polar and nonpolar blocks said blocks having predetermined sizes corresponding to their individual molar masses. These blocks of selected size render domains with a natural length scale associated with their respective molar masses and compositions. Further, by tuning the molar masses of the individual blocks within block copolymers, one can generate various morphologies with selected sizes, such as lamellae or cylinders of a specific width, specific pitch and specific symmetry patterns such as hexagonal close packed arrays or parallel lines.
Film layers made with energy neutral polymers (hereinafter, neutral layers) are sometimes used because they do not show preferred wetting for one of the polymer blocks over another and, therefore, tend not to enforce or guide, preferentially, the formation of a particular domain at a particular location. Neutral layers may be, functionalized polymer brushes, random copolymers having similar repeat units to those used in the block copolymer being used or blends of homopolymers, each respectively having similar monomers to those in the block copolymer being used.
Pinning layers are film layers made with polymers having a predominance of similar monomers to those in one of the blocks. These are sometimes used because they do show a preferred wetting for one of the polymer blocks over another and, therefore, tend to enforce or “pin,” preferentially, the formation of a particular domain at a particular location.
Among the methods used to guide self-assembly in thin films of block copolymers are graphoepitaxy and chemical epitaxy. In graphoepitaxy self-assembly is guided by pre formed topographical structures such as trenches. For example, a topographically patterned substrate with a neutral underlying surface and with sidewalls that are preferentially attracted to one type of the block copolymer domain (for example, the A domains of an A-B diblock copolymer assembly) can be used to direct self-assembly inside the trench through topographical confinement. With a trench of width L and a block copolymer (BCP) having a periodicity of PBCP, frequency multiplication of a factor of L/PBCP can be achieved for the remaining domain.
Various attempts have been made to incorporate crosslinking functionality into either a pinning layer or a neutral layer; the activation of which crosslinks the underlayer and retards comingling between the underlayer and the block copolymer. For example, in U.S. Pat. No. 8,226,838, Cheng et al. disclose an underlayer comprising “a cross linked organic polymer including an epoxy-based homopolymer or copolymer.” wherein epoxy based monomer repeat units include epoxydicyclopentadienyl methacrylate, glycidyl (meth)acrylate, 2,3-epoxycyclohexyl (meth)acrylate, (2,3-epoxycyclohexyl)methyl (meth)acrylate, 5,6-epoxynorbornene (meth)acrylate, and combinations comprising at least one of the foregoing. The epoxy based monomers are copolymerized with various other monomers, including styrene and methyl methacrylate, alone or in combination to provide, respectively, pinning layers or neutral layers as required. However, while the monomer repeat units listed above may work to provide a crosslinked substrate, they are less than effective in matching the interaction properties of monomers such as methyl methacrylate.
Therefore, there remains a need for polymer compositions that provide thermally crosslinkable curing and, at the same time provide surface interaction characteristics that resemble styrenic monomer repeat units.